It is desirable in text editing systems having a display screen to display the text information on the screen in a manner which replicates as far as possible the information which is to be printed on a document. Thus, where the characters are to be printed with proportional spacing it is desirable that the characters be displayed on the screen in a similar manner.
This is also the case where various character fonts are to be employed. It is desirable that the text editing equipment be capable of supporting a variety of different character sets. Examples of various character sets are pica, elite, mathematical, statistical, foreign language, scientific and proportional character sets.
The problem of providing a display which exactly replicates to the greatest extent possible the printed characters on a page and has a high resolution is compounded with a full screen display (that is, a display on which a full page of text is displayed at one time on a screen).
Within the above system it is further desirable, without the need to move data around which can cause loss of information, to be capable of scrolling in both vertical and horizontal directions.
Screen display systems for text editing systems have been utilized in the past. These systems have also incorporated the ability to handle specific character sets including proportional spacing. An example of such a system is shown in U.S. Pat. No. 4,054,948 granted to Grier et al and and U.S. Pat. No. 4,141,079 granted to the same inventors. This system involves the use of a fixed character set which is embodied in a read only memory (ROM) which is part of the word processing system. The ROM is part of the character generator and width generator. Coded typographic data is stored in the character memory. The dot format for the display of characters is in different scan lines being produced upon application of stored character data to the character generating ROM. This limits operation to only one particular character set at a time. Moreover, the system is limited to only four character widths. As a result the system can not be used with other character styles.
Another patent showing a text editing system with a screen capable of supporting a character set such as proportional characters is shown in U.S. Pat. No. 3,654,609 granted to Bluethman et al. This reference discloses an editing system including a CRT display which displays input characters in a proportionally spaced representation. The text character representations are stored in memory and are accessed by a processor. In this system, additional subsystems are attached to operate in conjunction with the single central processing unit. As additional subsystems are attached to the configuration to support additional users, the single processor begins to reach the limit of its capability. The response time for the central processor unit to respond to subsystem requests for data manipulation soon exceeds the time constraints for the additional subsystems. Moreover, should the single central processing unit fail, all of the subsystems are rendered inoperable and the entire system can not be utilized. The invention disclosed in this cited reference requires the use of a large memory with the single control processing unit. The single central processing unit itself is more expensive and more critical to the operation of the entire system than components used in distributred processing configurations.
U.S. Pat. No. 3,618,032 granted to Goldsberry et al also discloses the system described hereinabove. This reference discloses a full page screen system which shows a variety of characters sets. Vector strokes are used rather than raster scans on the CRT. In the vector stroke approach the characters on the CRT are formed by straight line segments which approximate the characters which are to be printed on the page. The characters thus do not necessarily closely represent the characters which are printed on the page.